Fuel cells (FCs) are versatile energy conversion technology. They can convert numerous high energy density fuels directly into electricity without first converting the chemical energy into thermal energy, bypassing the Carnot efficiency limitations of conventional heat engines. All conventional fuel cells consist of an anode where a fuel is oxidized and a cathode where an oxidant (typically oxygen) is reduced. The anode and cathode are separated by an ion transport membrane and connected via an external electrical circuit, as illustrated in FIG. 1A. FIG. 1B illustrates the equivalent circuit diagram used to represent the physical processes and test the FC with an applied voltage or source-measure unit. The free energy of reaction drives a DC current flow through a load in the external circuit while ions (typically H+, OH−, CO32−, or O2−) flow through the membrane. In addition to their potential for high efficiency, fuel cells may be operated with clean fuels such as hydrogen (yielding water as the only exhaust) or sustainable biofuels that are carbon neutral (produce no net CO2). Fuel cells come in many different varieties with a wide range of potential applications. High temperature fuel cells (solid oxide fuel cells, SOFCs) have typically been considered for stationary power, and low temperature fuel cells (proton exchange membrane fuel cells, or PEMFCs) have typically been considered for transportation or portable power. FCs yield high power density (especially when compared to technologies like solar energy). But still, market penetration for both PEMFCs and SOFCs is low due to high cost and reliability issues. The typical power density for a commercial PEMFC is around 600 mW/cm2, while that for an SOFC is around 300 mW/cm2. In 2009 the U.S. Department of Energy set fuel cell cost and lifetime goals to be $750/kW with 40,000 hours of operation for stationary power and $30/kW with 5,000 hours of operation for the transportation sector.
SOFC technology appears to be mature enough and poised for commercial success, particularly for stationary power applications where the valuable heat may be utilized and the net efficiency increased. The increased market penetration that now seems possible for SOFCs is actually a positive development for other fuel cells technologies as well. Some technologies have intrinsic advantages for applications such as transportation or portable power, but further development is hindered by the perception that FCs are problematic. The quick response time and low operating temperature are key for automotive and portable power applications. For these applications particular focus has been paid to PEMFCs. However there are many barriers to PEMFC development and commercialization. Scientific challenges still exist to improve the oxygen reduction reaction (ORR) catalyst, prevent poisoning of the hydrogen oxidation reaction (HOR) catalyst, and develop higher temperature proton conducting membranes. In addition there are more practical issues which need to be addressed like active water management to maintain membrane wetness without flooding, improving device reliability, and lowering system cost. Also, it could be viewed either as a liability or a great benefit, but the most advanced PEMFCs use hydrogen as the fuel. Below, we review some of the issues with conventional PEMFCs and review some novel fuel cell design concepts which seek to bypass the limitations of conventional devices, such as single-chamber and membraneless designs.
Conventional Proton-Exchange Membrane Fuel Cells (PEMFCs)
The oxidation of hydrogen to yield water is given by the following familiar reaction:
                    H                  2                      (            g            )                              +                        1          2                ⁢                  O                      2                          (              g              )                                            →                  H        2            ⁢              O                  (          l          )                                Δ      ⁢                          ⁢              G        n              =          237      ⁢              kJ        mol                        Δ      ⁢                          ⁢              E        n              =          1.23      ⁢                          ⁢      V                  Δ      ⁢                          ⁢              H        rxn        o              =          286      ⁢              kJ        mol            
This simple and clean reaction has tremendous appeal for using hydrogen as an energy carrier, especially when combined with renewable (solar or wind generated) hydrogen. The fly in the ointment that tempers one's enthusiasm for the fuel though is the difficulty with storing and distributing hydrogen. In addition, the low temperature fuel cell technology that converts it to electricity (the PEMFC) has several problems that have prevented its cost effectiveness and thus its widespread adoption. A summary of some of the most significant problems are presented below.
Catalysts for the Oxygen Reduction Reaction: High Activation Overpotential
The oxidation reduction reaction (ORR) is puzzling in that it is one of the oldest known electrochemical reactions, and yet it remains one of the most poorly understood. Numerous mechanisms have been proposed for the ORR on platinum, and the most widely accepted mechanism is the associative adsorption mechanism:O2→O2aas  (rxn1)O2ads+Hads++e−→HO2ads  (rxn2)HO2ads+Hads++e−→H2O+Oads  (rxn3)Oads+Hads++e−→HOads  (rxn4)HOads+Hads++e−→H2O  (rxn5)
The rate determining step is thought to be reaction rxn2 above, but the debate over mechanism and rate limiting step remains open. Increasing its reaction rate can dramatically improve fuel cell performance. Research on the ORR has focused mainly on the development of new catalysts. While the overall rate can also be increased by simply increasing the temperature beyond the 80° C. where PEMFCs are typically operated, this causes other issues, particularly with the ion transport membrane as discussed below.
Catalysts for the Hydrogen Oxidation Reaction: Poisoning
Carbon monoxide, sulfur, and other species can poison the HOR catalyst, typically platinum. This phenomenon is well known and has been recently reviewed. Since CO is a by-product of the reforming process through which most hydrogen is produced (FIG. 2), it is a serious impediment to the development of PEMFC technology. CO poisons a platinum catalyst by strongly chemisorbing to the surface and blocking the active reaction site. The easiest way to overcome the problem of CO poisoning is to increase the operating temperature. The tolerance of CO is directly related to temperature. Increasing the temperature to as little as 130° C. dramatically improves PEMFC performance in the presence of CO. However, as the temperature is increased, the PEM dries out and the series resistance increases leading to lower efficiency. The conventional approach is to develop an alternative to the Nafion polymer used in current technology or develop catalysts that are more tolerant to the presence of CO. However, other interesting ideas have been proposed. For instance, it has been proposed to inject oxygen or hydrogen peroxide into the fuel stream to oxidize CO before it reaches the catalyst. However, the poisoning issue is pernicious and unresolved.
Both ORR and HOR Catalysts: Degradation
Another problem with conventional PEMFC catalysts is degradation. Platinum particle dissolution and agglomeration and carbon support reaction are the main mechanisms of degradation, and both are facilitated by the presence of water. If the carbon support and liquid water were eliminated, many of the degradation concerns would be alleviated.
Membrane Humidification: Kinetics Better at Higher Temperature, but the Membrane Dries Out
PEMFC technology has as one of its primary advantages that it operates at a low temperature. However by increasing the temperature, fuel cell performance can be greatly improved. This is primarily due to the improvement in exchange current density, but performance improvements are also observed in efficiency due to better heat and water management. However the membrane in typical PEMFCs needs to be humidified for facile ion transport. This is because the proton conductivity of Nafion, the standard PEMFC membrane material, drops as the membrane dries causing an increase in ohmic losses and ultimately leads to device failure. To get around this problem it is necessary to have an external humidifier to run PEMFCs at higher temperatures. Also, because they have a hydrated membrane, PEMFCs can also suffer catastrophic failure in sub-freezing environmental conditions. Getting around these limits requires the development of new membrane materials, which is a highly active field of study. However at present no alternative membrane technologies have both high performance and robust operation at high temperature. This issue with proton exchange membranes has prompted the development of several unconventional fuel cell designs.
Unconventional Fuel Cell Designs
In order to circumvent many of the issues discussed above, entirely new device architectures have been proposed. These include so called “single-chamber” designs and “membraneless” designs. A summary of a few of these devices is provided below.
Single-chamber fuel cell concepts (FIGS. 3A and 3B) were considered as early as the late 1950s. But in 1990, a single-chamber design generated both interest and controversy. The device functions similar to a conventional PEMFC with the major exception that the hydrogen and oxygen are mixed and fed to the same side of the device. The mechanism of operation has been debated, but the devices were able to achieve about 1 volt and power densities of 1 to 5 mW/cm2. The design has the advantage that it does not need seals and could be fabricated in a simple manner. However, since hydrogen is also present at the outer electrode, there is a substantial chemical (not electrochemical) reaction rate between hydrogen and oxygen, which represents a significant loss in efficiency. Interestingly, it was noted that high humidification and low pressures reduced this undesirable side reaction. Others later developed a two sided design with selective catalysts (FIG. 3B) that operates more similar to an SOFC. This latter device also generated significant interest and spawned several research efforts.
Another unconventional design uses mass transport limitations to keep fuel away from the cathode and the oxidant away from the anode. These “membraneless” designs do not have a membrane, but they still require the DC transport of an ion—typically diffusion across a laminar flow field in a microfluidic channel. The fuel and oxidant streams are merged at a y-junction (FIG. 4) at low Reynolds number (Re<10). As the fluid flows down the channel, the fuel and oxidant species begin to diffuse across the channel creating a diffusion zone which acts quite similar to a membrane in a conventional fuel cell. One advantage of this design is its simplicity and compact size, but they yield very low power densities, have very poor fuel efficiency, and require continuous liquid flow (and perhaps recycling).
In short, both of these unconventional fuel cell designs have some unique traits, but neither addresses the key issues with low temperature fuel cell technology without creating more serious problems (such as low faradic efficiency). The high cost of platinum and Nafion, the low temperature HOR poisoning, ORR and HOR catalyst degradation, the presence of liquid water, and the difficulty of actively managing water all combine to provide significant technical and reliability barriers that inhibit PEMFC technology.
A solution that avoids these issues would be a breakthrough that would significantly alter and enhance the prospects for low-temperature fuel cell technology.